This invention relates to a thermoplastic heat-fusion pipe fitting system and a method of electro-welding thermoplastic pipe using such a system. Thermoplastic heat-fusion pipe fittings have long been used to join thermoplastic pipe members. There are many designs for such fittings, which tend to fall into three basic types: (1) those having an electrical heating element which is embedded within the body of the fitting and located in close proximity to the surface of the fitting that is to be joined to the pipe member by heat-fusion for an example, as disclosed by Bauer et al. in U.S. Pat. No. 3,432,179; (2) a metal core embedded in the body of the fitting in which eddy currents can be induced by the action of an induction coil thereby producing a heating effect in the core; and (3) those which are meant to be heated by the application of an external heat source.
Each design of heat-fusion pipe fitting requires a special amount of heat to be delivered to it to effect fusion of the fitting to a pipe member. For example, each variation in the design of an electrical heating element used in a type (1) heat-fusion pipe fitting changes its electrical characteristics, and thus the amount of power developed in the heating element to produce sufficient heat to effect fusion. Since most of these pipe fittings are installed in a trench, this variation in heat requirements can cause problems in field installation and often necessitates using labor having a greater degree of skill than is normally required for installing thermoplastic pipe lines and pipe line replacement sections.
In order to simplify installation of type (1) pipe fittings, Blumenkranz in U.S. Pat. No. 3,465,126 discloses a power unit for use in thermal welding of thermoplastic pipe sections. This power unit would be used to supply the energy to the electrical heating element under controlled conditions as to voltage and duration of power application. Predetermined levels of output voltage and power application are set prior to the use of the power unit in the field. However, this type of procedure does not make allowances for field use of different types of fittings on the same job site.
Published European Patent Application No. 76 043 discloses a type (1) electrofusion pipe fitting having incorporated therein an identity resistor. This resistor would have a value dependent on the energy to be dissipated by the electrical heating element within the fitting. This fitting would be used in conjunction with control apparatus that would sense the characteristics of the identity resistor and use this information to automatically control the power applied to the electrofusion fitting. However, the fitting employed in this system is more difficult to manufacture than one having an electrical heating element alone, because of the addition of the identity resistor and its associated wiring and terminals.